In recent years, resins formed of plant-derived materials have been drawing attention from the viewpoint of environmental conservation. Among the resins formed of plant-derived materials, polylactic acid is one of the resins having the highest heat resistance and capable of mass production, so polylactic acid can be obtained at a low cost and is highly useful. The recent applications thereof varies widely from applications having short period of service time based on an assumption that the resin will be discarded, e.g., a container and packing and an agricultural film, to highly functional applications which can retain initial properties for a long period of time, e.g., housings of home electric appliances and OA equipment and automobile parts.
Polylactic acid is a crystalline resin, and it is important that the resin be crystallized in order to exhibit the original material properties thereof, such as heat resistance. However, polylactic acid has a low crystallization rate, so the crystallization thereof does not proceed sufficiently when molded in a short period of time and has tendencies that the heat resistance, elastic modulus, and the like thereof decrease.
Accordingly, as a method of improving the moldability of polylactic acid, a method of increasing the crystallization rate of polylactic acid has been proposed. For example, in Japanese Patent No. 3411168 (Patent Document 1), it is described that a molded article retaining its transparency and crystrallizability can be obtained by adding a transparent nucleating agent such as aliphatic amide to aliphatic polyesters, such as polylactic acid.
Further, Japanese Unexamined Patent Application Publication No. 2003-226801 (Patent Document 2) describes that the crystallization rate of polylactic acid is significantly increased by adding a low molecular compound having an amide group and a layered clay mineral organized with an organic onium salt to polylactic acid.
Further, in order to increase the crystallization rate, a method of adding a (meth)acrylate compound to polylactic acid is also disclosed (for example, see Japanese Unexamined Patent Application Publication No. 2003-128901 (Patent Document 3)).
When the transparent nucleating agent, such as aliphatic amide, exemplified in Patent Document 1 is added to polylactic acid, an increase in crystallization rate of polylactic acid can be observed compared with the case of an additive-free polylactic acid. However, the effect of the addition is not sufficient, therefore, in order to obtain a molded article having a sufficient crystallinity, a heat treatment after molding is required. Further, because a crystallinity of polylactic acid is low, the crystal solidification in a mold at the time of, for example, injection molding, is tend to be insufficient. As a result, there are some defects such that the molded article is deformed at the time of mold releasing.
Further, when a low molecular compound having amide group and a layered clay mineral organized with an organic onium salt disclosed in Patent Document 2 are added to polylactic acid, an increase in crystallization rate and an improvement in moldability of polylactic acid can be observed compared with the cases of the additive-free polylactic acid and polylactic acid being added with the nucleating agent such as aliphatic amide, but the effects thereof are insufficient. Further, in the resin composition described in Patent Document 3, the crystallization rate is increased, but the effects thereof are not necessarily sufficient in some cases.